1. Field of the Invention
The present invention relates to a storage medium having a game program stored thereon and a game apparatus, and more particularly to a rendering process for a background image in a virtual game space.
2. Description of the Background Art
In recent years, a three-dimensional game using a three-dimensional computer graphics technology has been widespread with a computer incorporated in a game system having an enhanced performance. In the three-dimensional game, objects of polygons such as a player object and topography objects are disposed in a three-dimensional game space so as to represent, on a three-dimensional game screen, a state of the game space seen from a predetermined point of view.
As a rendering method for rendering a background in a player's (virtual camera's) field of vision in a game for allowing the player to operate a player character in the aforementioned virtual space, the following method can be used. In the method, an α value is set for each pixel in accordance with a depth value (Z value) of the pixel of an original image rendered as a background. A blurred image corresponding to the original image is generated. The original image is combined with the blurred image based on the α value so as to render the combined image. This method is disclosed in, for example, Japanese Laid-Open Patent Publication No. 2001-175884 (hereinafter, referred to as Patent Document 1).
In the rendering method disclosed in Patent Document 1, a combination ratio between the original image and the blurred image can be changed against the depth value, whereby it is possible to represent a depth of field and the like. However, such a method fails in rendering, with reality, the background which quickly changes in accordance with the character being operated in the virtual space. Specifically, for example, when a person quickly turns backward, the background quickly changes, so that he or she cannot quickly focus his or her eyes in the real world. That is, he or she gradually focuses his or her eyes as time passes. Human eyes have such characteristics. However, the rendering method disclosed in Patent Document 1 is not based on such characteristics of human eyes. The combination ratio between the original image and the blurred image changes against only the depth value. Therefore, when the depth value is unchanged, the constant depth of field is used to render the background regardless of how the background is changed. Accordingly, the image lacks reality when seen with human eyes.